In various applications requiring operations on a web of material (film) a web must be transported past an operating station, usually from a web supply to a web take-up mechanism. The web of material may assume diverse configuration dependent upon the particular use thereof and the operations performed thereon. Thus, the web may comprise a film strip upon which information is recorded and retrieved, such as microfilm.
Prior art transport devices may utilize a driven capstan adapted for bi-directional operation to pull the film from a supply reel to a take-up reel. A reversible motor mechanically coupled to the capstan is controlled by a servo system to dictate the direction of travel of the tape, to maintain a substantially constant linear velocity of the film through the processing station, to effect speed changes where required or upon command and to introduce substantial accelerations to the transported film, e.g., during start and stop operations. For most, if not all, of the foregoing controller operations, it is necessary that a substantially constant tension be maintained in the film to minimize the errors attending the recording and retrieval of information, and to minimize wear and tear of the film itself. The film tension achieved in capstan drive film transport systems has generally been adequate. Many of the systems which are designed to provide constant web tension in a direct drive reel-to-reel system also provide means for maintaining the velocity of the web at a substantially constant value. If a high resolution shaft encoder, or tachometer, is utilized in the film path to sense film speed, proper tension is applied to the film to avoid slippage between the film and shaft encoder.
In many applications, such as the searching and retrieval of information from a selected frame between the beginning and end portions of the film, the film being driven in a stepping type of motion, it is required to stop the film at the exact position it occupies when an event occurs, like the detection of a desired frame position. This stopping is primarily accomplished by applying decelerating forces to the capstan. The behavior of idler rollers and dancer arms (if used) located downstream from the driven capstan is usually well behaved, since the stopping action increases the local film tension. The maximum torque deliverable to the upstream rollers is limited to a value proportional to the film tension, which is locally reduced during the stopping action. If the stopping is too abrupt, negative film tension may be generated. This will cause the film to loop away from its designed path. This looping may damage the film if it strikes nearby objects. Additionally, an abrupt restoration of positive film tension after negative tension has been applied to the film may break the film.
Further, looping of the film allows differential speeds to develope between the film and whatever guiding rollers are in the looping region, these differential speeds tending to cause the film to be scratched when positive film/roller contact is re-established. In any of the above situations, the film will generally overshoot the desired position by an undetermined amount, thereby making the further positioning of the film difficult. Thus the film transport designer must limit the acceleration that may be imparted to the film. The benefit is that the film is handled smoothly, and the overshoot becomes defined. The attendant penalty is that the overshoot becomes large.